The cannabis plant's flowers contain a chemical or drug known as THC (short for tetra-hydro-cannabinol). Smoking or eating the flower can make a person feel euphoric (very good) or sleepy. The plant is also used to make hemp fibre, and for its seeds and seed oil.

When a person inhales cannabis smoke or consumes cannabis, he or she may get a feeling called "getting high" or "getting stoned". Cannabis's most common effects include feeling happy, relaxed, tired, silly or scared; having many ideas about what to do; not being able to think clearly (or remember some things at all); and getting hungry (also called getting "the munchies"). Smoking cannabis changes how people think and feel, making it either harder or easier to solve some problems. Some people who take cannabis feel strange or paranoid (worried that something bad is going to happen).

Hashish (dried resin) is much more concentrated than cannabis (it includes both leaves and flowers). Because of this, people who take large amounts of hashish may feel even stronger effects. They may also see things or hear things that do not exist (these are called hallucinations). They may also have strange thoughts. Some hashish users like the feeling of these visions and thoughts, while others may find them scary. However, people rarely get hallucinations after smoking or eating cannabis.

On December 11, 2013, Uruguay was the first country in the world to make it legal to grow, sell, and use cannabis for personal (or recreational) use.[10] Other countries that allow people to have small amounts of cannabis (just enough for them to use personally) include Colombia, Ecuador, Jamaica, the Netherlands, Peru, Portugal, Spain, and Switzerland.[11] In the United States, on January 1, 2014, the states of Washington and Colorado made it legal for people aged 21 or older to buy cannabis.[12] In November 2016, California did the same thing, and has been selling cannabis in stores since January 2018.[13] Later in 2018, Canada will become the first G7 nation to make cannabis legal for anybody,[14] although each province can decide on how much of it is allowed.[15]

Cannabis is one of the least dangerous of the commonly used illegal drugs. It is almost impossible to overdose on cannabis. A person would have to smoke their entire body weight in five minutes in order to overdose.[16][17][18]

As of 1995, scientists had not shown that smoking cannabis causes lung cancer, even in people who smoke a lot of cannabis for a long time.[19][20]

It is a common belief that people who use cannabis are less interested in life and may not want to go to school or work (scientists call this "amotivational syndrome"). However, many reports show that people who use cannabis do just as well as non-users.[21][22]

A person who is intoxicated ("high" or "stoned") from cannabis could get hurt or killed in an accident if they drive a car. It is not safe to drive under the influence of any intoxicant. However, drivers who are "stoned" are much less likely to get into car accidents than drunk drivers and in some cases, sober drivers.[23][24]

"Hard drugs", such as heroin, meth, and cocaine, are chemically addictive. This means that if a person starts taking heroin, meth, or cocaine, that person's body will physically need to keep taking the drug. If they try to stop using the drug, they may become very sick. Cannabis and hashish are not chemically addictive, but they can be psychologically or habitually addictive. This means that people can get so used to the pleasure cannabis causes that they feel as if they need the drug. Unlike with alcohol, tobacco, and hard drugs, most people who use cannabis can stop taking it when they want, while experiencing only minor withdrawal symptoms. However, while cannabis may not be as addictive as other drugs, people can still become very much addicted to the pleasure of cannabis.[25]